U.S. patent application number 10/808959 was filed with the patent office on 2004-12-02 for time-variable magnetic fields generator for a magnetic resonance apparatus.
Invention is credited to Heid, Oliver, Vester, Markus.
Application Number | 20040239327 10/808959 |
Document ID | / |
Family ID | 33311740 |
Filed Date | 2004-12-02 |
United States Patent
Application |
20040239327 |
Kind Code |
A1 |
Heid, Oliver ; et
al. |
December 2, 2004 |
Time-variable magnetic fields generator for a magnetic resonance
apparatus
Abstract
A time-variable magnetic fields generator for a magnetic
resonance apparatus has at least one gradient coil with conductors
extending essentially in the region of a subject-receiving hollow
opening of the magnetic resonance apparatus, and that is free of
conductors in a middle axial region of the hollow opening, a first
radio-frequency shield that encloses the conductors disposed on the
one side of the middle region, a second radio-frequency shield that
encloses the conductors disposed on the other side of the middle
region, a radio-frequency antenna element that emits a
radio-frequency field, disposed between the first and second
radio-frequency shield in the middle region, a third
radio-frequency shield proceeding radially, externally around the
antenna element, such that the radio-frequency shields delimit a
field return space within the generator and that is designed for a
return of the radio-frequency field.
Inventors: |
Heid, Oliver; (Gunzenhausen,
DE) ; Vester, Markus; (Numberg, DE) |
Correspondence
Address: |
SCHIFF HARDIN, LLP
PATENT DEPARTMENT
6600 SEARS TOWER
CHICAGO
IL
60606-6473
US
|
Family ID: |
33311740 |
Appl. No.: |
10/808959 |
Filed: |
March 25, 2004 |
Current U.S.
Class: |
324/318 |
Current CPC
Class: |
G01R 33/422 20130101;
G01R 33/34046 20130101; G01R 33/3415 20130101; G01R 33/34076
20130101 |
Class at
Publication: |
324/318 |
International
Class: |
G01V 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 2003 |
DE |
103 13 229.5 |
Mar 11, 2004 |
DE |
10 2004 012 058.7 |
Claims
We claim as our invention:
1. A time-variable magnetic fields generator for a magnetic
resonance apparatus comprising: a gradient coil formed by
conductors adapted to be disposed at a subject-receiving, hollow
opening of a magnetic resonance apparatus, said hollow opening
having an axial extent and said gradient coil being free of said
conductors at an axial middle region of said hollow opening; a
radio-frequency antenna element for emitting a radio-frequency
field, adapted to be disposed in said middle region; a first
radio-frequency shield enclosing said conductors at a first side of
said middle region; a second radio-frequency shield enclosing said
conductors at a second, opposite side of said middle region; a
third radio-frequency shield proceeding radially around an exterior
of said radio-frequency antenna element; and said first, second and
third radio-frequency shields delimiting a field return space for
return of said radio-frequency field.
2. A generator as claimed in claim 1 wherein said gradient coil is
adapted to be disposed at a subject-receiving hollow opening
wherein said middle region is cylindrical.
3. A generator as claimed in claim 2 wherein said radio-frequency
antenna element is adapted to extend into said hollow opening.
4. A generator as claimed in claim 1 wherein said gradient coil is
adapted to be disposed at a subject-receiving hollow opening
wherein said middle region is barrel-shaped.
5. A generator as claimed in claim 1 wherein said radio-frequency
antenna element connects said first and second radio-frequency
shields together in terms of radio-frequency, and wherein said
first and second radio-frequency shields in combination with said
radio-frequency antenna element form a radio-frequency antenna.
6. A generator as claimed in claim 5 wherein said first and second
radio-frequency shields and said radio-frequency antenna elements
form a birdcage antenna, as said radio-frequency antenna.
7. A generator as claimed in claim 5 wherein said first and second
radio-frequency shields and said radio-frequency antenna elements
form an array antenna, as said radio-frequency antenna.
8. A generator as claimed in claim 1 comprising a connection
connecting each of said first and second radio-frequency shields
with said third radio-frequency shield, said connection also
forming a radio-frequency shield, for causing said first, second
and third radio-frequency shields and said connection to shield
said return space from radio-frequency up to said middle
region.
9. A generator as claimed in claim 1 wherein said first, second and
third radio-frequency shields in combination with said
radio-frequency antenna element form a radio-frequency antenna in
which said third radio-frequency shield is a return conductor.
10. A generator as claimed in claim 1 wherein said gradient coil
generates a gradient field, and wherein at least one of said first,
second and third radio-frequency shields is permeable for said
gradient field and is substantially impermeable for said
radio-frequency field.
11. A generator as claimed in claim 1 comprising a gradient
shielding coil associated with said gradient coil.
12. A generator as claimed in claim 11 wherein said gradient
shielding coil is outwardly radially spaced from said gradient
coil, and wherein said third radio-frequency shield is disposed
between said gradient shielding coil and said gradient coil.
13. A generator as claimed in claim 12 wherein said gradient coil,
said first and second radio-frequency shields and said
radio-frequency antenna element comprise a unitary structural
component.
14. A generator as claimed in claim 1 wherein said gradient coil
system is comprised of two halves, and wherein said radio-frequency
antenna element is disposed between said two halves.
15. A magnetic resonance apparatus comprising: a housing containing
a hollow opening adapted to receive an examination subject therein,
at least partially surrounded by a basic field magnet for
generating a basic magnetic field in an imaging volume within said
hollow opening; and a time-variable magnetic fields generator
comprising a gradient coil formed by conductors adapted to be
disposed at said hollow opening of a magnetic resonance apparatus,
said hollow opening having an axial extent and said gradient coil
being free of said conductors at an axial middle region of said
hollow opening, a radio-frequency antenna element for emitting a
radio-frequency field, adapted to be disposed in said middle
region, a first radio-frequency shield enclosing said conductors at
a first side of said middle region, a second radio-frequency shield
enclosing said conductors at a second, opposite side of said middle
region, a third radio-frequency shield proceeding radially around
an exterior of said radio-frequency antenna element, and said
first, second and third radio-frequency shields delimiting a field
return space for return of said radio-frequency field.
16. A magnetic resonance apparatus as claimed in claim 15 wherein
said conductors of said gradient coil are adapted to carry a
time-varying current for generating a gradient field and wherein
said housing contains elements that interact with said gradient
field and generate an eddy current, with an associated eddy current
field, said eddy current field causing said gradient field to have
a non-linear portion in said imaging volume, and wherein said
magnetic resonance apparatus comprises an electrically conductive
structure in said housing at least partially surrounding said
gradient coil for, triggered by a change in said current carried by
said conductors of said gradient coil, generates a compensating
eddy current field in said imaging volume for compensating said
non-linear portion.
17. A magnetic resonance apparatus as claimed in claim 16 wherein
said gradient coil and said electrically conductive structure are
tuned to each other for causing said electrically conductive
structure to generate a compensating eddy current field that is
geometrically similar to said gradient field.
18. A magnetic resonance apparatus as claimed in claim 16 wherein
said electrically conductive structure is a portion of said basic
field magnet.
19. A magnetic resonance apparatus as claimed in claim 18 wherein
said basic field magnet comprises a vacuum vessel, and wherein said
vacuum vessel is said electrically conductive structure.
20. A magnetic resonance apparatus as claimed in claim 16 wherein
said electrically conductive structure is barrel-shaped.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention concerns a generator of time-variable magnetic
fields for a magnetic resonance device with at least one gradient
coil, with conductors of the gradient coil extending substantially
in the region of a hollow cylinder, and with the gradient coil
being free of conductors in a middle region along the axial extent
of the hollow cylinder, and a magnetic resonance device with such a
generator.
[0003] 2. Description of the Prior Art
[0004] Magnetic resonance technology is a known modality to, among
other things, acquire images of the inside of a body of an
examination subject. In a magnetic resonance device, rapidly
switched gradient fields are generated by a gradient coil system
and are superimposed on a static basic magnetic field that is
generated by a basic field magnet. Furthermore, the magnetic
resonance device has a radio-frequency system that radiates
radio-frequency signals into the examination subject to excite
magnetic resonance signals and acquires the excited magnetic
resonance signals, on the basis of which magnetic resonance images
are generated.
[0005] To generate gradient fields, appropriate currents are set in
gradient coils of the gradient coil system. The amplitudes of the
required currents are up to more than 100 A. The current rise and
fall rates are up to more than 100 kA/s. Since the gradient coil
system normally is surrounded by electrically conductive
structures, eddy currents are induced in these by the switched
gradient fields. Examples for such conductive structures are the
vacuum vessel and/or the cryoshield of a superconducting basic
field magnet. The fields arising as a consequence of the eddy
currents are undesirable because, without counter measures, they
weaken the gradient fields and distort them with regard to their
time curve, which leads to impairment of the quality of magnetic
resonance images.
[0006] The distortion of a gradient field as a result of the eddy
current fields can be compensated up to a certain degree by a
corresponding pre-distortion of a quantity used for controlling the
gradient field. The eddy currents induced on a predetermined
enveloping surface (that, for example, runs through an inner
cylinder jacket of an 80-K cryoshield of the superconducting basic
field magnet) by the gradient coils being fed with current also can
be reduced by the use of an actively shielded gradient coil system.
A gradient shielding coil associated with the gradient coil
normally has a lower number of windings than the gradient coil, and
is connected with the gradient coil such that the same current that
flows through the gradient coil flows through the gradient
shielding coil, but in the opposite direction. The gradient
shielding coil thereby has a weakening effect on the gradient field
in the imaging volume.
[0007] Furthermore it is known from German OS 3445724 to minimize
magnetic coupling between an RF coil and a gradient field coil, for
example by arranging shielding layers on both sides of the gradient
field coil.
[0008] A magnetic resonance device is known from German OS 44 14
371 in which a radio-frequency shield is arranged between the
radio-frequency antenna and the gradient coil system of the
magnetic resonance device, the radio-frequency shield being
permeable for the electromagnetic fields generated by the gradient
coil system in the low-frequency range and impermeable for the
fields generated by the radio-frequency antenna in the
radio-frequency range. The radio-frequency shield has a first
electrically conductive layer arrangement and a second electrically
conductive layer arrangement arranged oppositely thereto that are
separated from one another by a dielectric. The layer arrangements
have adjacently arranged conductor tracks that are separated from
one another by electrically insulted grooves; the grooves being
offset in the first layer arrangement compared with the second; and
in at least one of the layer arrangements, adjacent conductor
tracks are connected with one another via specially arranged
bridges, for example formed by capacitors, that conduct
high-frequency currents.
[0009] The radio-frequency antenna of the magnetic resonance device
may be fashioned as a so-called birdcage antenna. A birdcage
antenna normally is fashioned to generate a homogenous
radio-frequency field within a volume enclosed by it, with
conductors that are parallel to one another and equally separated
being connected with one another via ferrules and defining a
cylindrical surface. Tuning is accomplished in the high-pass and
low-pass filter ranges by capacitors in each of the conductors, or
in the ferrules between the conductors, such that a homogenous
radio-frequency field results upon resonance. Embodiments of such a
birdcage antenna are found, for example, disclosed in U.S. Pat. No.
4,680,548. The radio-frequency antenna also can be fashioned as an
array antenna. The array antenna is characterized by a number of
essentially uniform, mutually overlapping conductor loops.
Embodiments of such an array antenna are disclosed, for example, in
U.S. Pat. No. 4,825,162.
[0010] A magnetic resonance device with a gradient coil system is
known from German OS 101 56 770, in which an
electrically-conductive structure is arranged and fashioned such
that, at least within an imaging volume of the magnetic resonance
device, a magnetic field of the structure caused by a gradient
field via induction effects is similar to the gradient field. In an
embodiment, at least one part of the structure is fashioned
barrel-shaped as a component of a basic field magnet. Among other
things, the gradient coil system can be fashioned without gradient
shielding coils, since the undesirable results of the switched
gradient fields (due to the similarity of the magnetic field caused
by the structure) can be almost completely controlled by a
pre-distortion, such that no weakening of the gradient fields
occurs due to the gradient shielding coils.
[0011] An MR device is known from German OS 4230145 that has a
basic field magnet that enables a transverse access to the
measurement volume. The MR device has a gradient coil system with
axially separated segments. To generate an essentially homogenous
RF field in the measurement volume, an axial RF coil system is used
that can be introduced into an axial bore of a supporting body or
transversally into the recess of the basic field magnet. The MR
device or, respectively, its components (such as the basic field
magnet, gradient coil system and RF coil system) are fashioned to
achieve an optimally large access to the measurement volume for
simple implementation of therapy measures such as microsurgical
operations, etc.
[0012] An MR device is known from U.S. Pat. No. 4,864,241 in which
eddy currents are compensated. This ensues by the use of two-part
gradient coils that typically form a hollow-cylindrical unit. For
RF field generation, a likewise hollow-cylindrically fashioned RF
antenna with smaller radius is introduced into the gradient coil
unit. Such a design has the disadvantages that it requires a
significant amount of space, and that the examination volume of the
MR device is determined by the diameter of the RF antenna.
SUMMARY OF THE INVENTION
[0013] An object of the present invention is to provide a generator
of time-variable magnetic fields of a magnetic resonance device,
and a magnetic resonance device with such a generator, in which a
device volume that surrounds a predetermined space for exposure of
an examination subject can be kept to optimally small
dimensions.
[0014] This object is achieved in accordance with the invention by
a time-variable magnetic fields generator for a magnetic resonance
device having at least one gradient coil with conductors extending
essentially in the region of a subject-receiving hollow opening of
a magnetic resonance apparatus and that is fashioned free of
conductors in a middle axial region of the hollow opening, a first
radio-frequency shield that encloses the conductors disposed at one
side of the middle region, a second radio-frequency shield that
encloses the conductors disposed at the other side of the middle
region, a radio-frequency antenna element that emits a
radio-frequency field and that is disposed between the first and
second radio-frequency shield in the middle region, and a third
radio-frequency shield that proceeds radially, externally around
the antenna element, such that the radio-frequency shields confine
a field return space that is within the generator and that is
fashioned for return or reverse propagation of the radio-frequency
field.
[0015] Due to the inventive design of the generator, a region (not
provided in conventional solutions) for return of the
radio-frequency field (generated by the radio-frequency antenna
element) is provided as a field return space within the gradient
coil system having the gradient coil. In contrast to the comparable
conventional solutions, the structural combination of the gradient
coil system and the radio-frequency antenna can be designed with a
smaller external diameter, given a consistent internal diameter, or
with a larger internal diameter given a consistent external
diameter. In the first case, the basic field magnet of the magnetic
resonance device can be dimensioned smaller and thus substantially
less costly. In the second case, given an unchanged basic field
magnet a larger examination subject acceptance space is achieved
that, among other things, increases the patient comfort. These
advantages of the invention result, among other things, from the
arrangement of the RF antenna element in the middle region, meaning
between the first and second RF shields, and thus the space
occupied by the gradient coil is optimally (doubly) used.
[0016] A further advantage of the use of a field return space
integrated (preferably sealed) in the generator is that the RF
field, at least in this region, is uninfluenced by external
activities. This allows the RF field to be monitored and, using the
monitoring result, more precisely generated. The third RF shield
preferably is disposed not only in the region of the antenna
element, and thus in the region of the strongest RF field, but also
extends on both sides in the axial direction in regions that are
radially disposed outside of the gradient coils surrounded by the
RF shields.
[0017] In an advantageous embodiment of the generator, the
radio-frequency antenna element connects the first and second
radio-frequency shields in terms of radio-frequency, such that the
first and the second radio-frequency shields, together with the
radio-frequency antenna element, form a radio-frequency antenna.
This has the advantage that substantial conductor sections of the
RF antenna can be formed by conductors (the RF shields) that are
anyway present for the gradient coil shielding. This leads to a
more compact design of the generator due to the high degree of
structural integration of the components used, especially the
double use of the RF shields. The connection in terms of
radio-frequency can be galvanic or non-galvanic, since in both
cases the currents generating the RF field propagate essentially
identically in the RF shields. The antenna element typically has a
feed-in for an RF signal for RF field generation and/or a read-out
for a received MR signal.
[0018] In a further advantageous embodiment, the first and second
radio-frequency shields are each connected to the third
radio-frequency shield via a connection likewise functioning as a
radio-frequency shield, such that the field return space is
shielded from radio-frequency up to the middle region. This has the
advantage that, in the axial direction, electrical conductors can
be displaced lateral to the field return space that also (without
further measures vis--vis the RF field) do not interact with the RF
field. Thus, for example, the gradient coil and a shielding coil
associated with it can be connected in series without an inductance
in the electrically connected conductors. In special embodiments,
the radio-frequency connection of the RF shields can be effected
by, for example, the first and/or second RF shield being arranged
at least in a region near to the third RF shield, or by the first
and/or second RF shield being galvanically connected with the third
RF shield via a further RF shield.
DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a longitudinal section through a magnetic
resonance device with a tunnel-like patient acceptance chamber
according to the prior art.
[0020] FIG. 2 is a longitudinal section through a magnetic
resonance device with a tunnel-like patient acceptance chamber and
with an actively shielded gradient coil system with an integrated
radio-frequency antenna in accordance with the invention.
[0021] FIG. 3 shows a magnetic resonance device corresponding to
FIG. 2, with a field return space shielded on all sides from
radio-frequency, with the exception of the middle region in
accordance with the invention.
[0022] FIG. 4 is a longitudinal section through a magnetic
resonance device with a tunnel-like patient acceptance chamber,
with a basic field magnet with a barrel-shaped cavity, and with a
non-actively shielded gradient coil system formed by two halves,
between which a radio-frequency antenna is arranged in accordance
with the invention,
[0023] FIG. 5 shows a magnetic resonance device corresponding to
FIG. 4, wherein RF shields that surround the gradient coils and an
RF shield situated radially outside the gradient coils are
spatially adjacent to one another in regions at the edge in the
axial direction, such that, with the exception of the middle
region, a field return space shielded from radio-frequency is
present in accordance with the invention.
[0024] FIG. 6 shows the radio-frequency antenna of FIGS. 2 through
5 formed as a birdcage antenna.
[0025] FIG. 7 shows the radio-frequency antenna of FIGS. 2 through
5 formed as an array antenna.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] FIG. 1 shows a longitudinal section through an upper half of
a magnetic resonance device with a tunnel-shaped patient acceptance
chamber according to the prior art, wherein for clarity only the
components in the sectional plane are shown. The magnetic resonance
device has an essentially hollow-cylindrical basic field magnet 110
that, to generate an optimally homogenous static basic magnetic
field in the patient acceptance chamber, has superconducting
primary coils 114 and likewise superconducting shielding coils 115
associated with the primary coils 114.
[0027] A likewise essentially hollow-cylindrical gradient coil
system 120 to generate rapidly switchable gradient fields is
arranged in the cavity of the basic field magnet 110. The gradient
coil system 120 includes, diagnosed from the inside out, the
following elements that are concentric to one another in
essentially hollow-cylindrical sub-regions of the gradient coil
system 120: a first transverse gradient coil 121, formed by four
saddle coils to generate a first transverse gradient field with a
gradient in a direction perpendicular to the hollow-cylinder main
axis 150; a second transverse gradient coil 122, likewise formed by
four saddle coils to generate a second transverse gradient field
with a gradient in a direction perpendicular to that of the first
transverse gradient coil 121 and perpendicular to the
hollow-cylinder main axis 150; a cooling device (not shown) to cool
the gradient coils 121, 122 and 123; a longitudinal gradient coil
123 formed by two solenoid coils to generate a longitudinal
gradient field with a gradient in the direction of the
hollow-cylinder main axis 150; a further cooling device in
connection with a shim device that are not shown; a longitudinal
gradient shielding coil 127 associated with the longitudinal
gradient coil 123; a first transverse gradient shielding coil 125
associated with the first transverse gradient coil 121, and a
second transverse gradient shielding coil 126 associated with the
second transverse gradient coil 122.
[0028] Since the conductor structures of the gradient coil system
120 are comparatively large and significantly lossy for many
wavelengths in the radio-frequency range, an essentially
hollow-cylindrical radio-frequency shield 130 is disposed between
the gradient coil system 120 and a radio-frequency antenna 140.
This radio-frequency shield is fashioned such that passes the
gradient fields generated by the gradient coil system 120 in a
low-frequency range and is impermeable for the signals generated by
the radio-frequency antenna 140 in the high-frequency
(radio-frequency) range.
[0029] The radio-frequency antenna 140 is disposed in the cavity of
the radio-frequency shield 130 formed as a birdcage antenna in the
illustration. A radio-frequency field can be generated in the
patient acceptance chamber with the radio-frequency antenna 140,
with exemplary field lines 149 of the radio-frequency field being
indicated in the region of the patient acceptance chamber with the
symbol {circle over (.multidot.)}. The symbol {circle over
(.multidot.)} thereby identifies a numbered field line 149 exiting
from the drawing plane at this location. The actual radio-frequency
antenna 140 is separated, for example, by approximately 3 cm from
the radio-frequency shield 130. Compared to a predetermined size of
the patient acceptance chamber for a basic field magnet not taking
into account the 3 cm, these 3 cm represent a dimensional
enlargement of approximately 10% that has significant costs. This
separation enables a flux return of the radio-frequency field
generated by the radio-frequency antenna 140, thus a closing of the
field lines 149, with the field lines being indicated with the
symbol {circle over (.times.)} in the region between the
radio-frequency antenna 140 and the radio-frequency shield 130. The
symbol {circle over (.times.)} identifies a numbered field line 149
entering the drawing plane at this location. The width of this
space provided for the flux return cannot be selected too small,
since otherwise the counter-propagating portions of the field lines
149 would lie very close to one another, and an unacceptably large
portion of the field energy would be located in the return, and the
filling factor and the efficiency of the radio-frequency antenna
150 would be significantly decreased.
[0030] Furthermore, a field line 119 of the basic magnetic field is
shown as an example in FIG. 1 that encloses the region of the basic
field magnet 110, and exemplary field lines 129 of the second
transverse gradient field are shown that enclose the region of the
gradient coil system 120. All magnetic fields that are applied in
the patient acceptance chamber must close outside of the patient
acceptance chamber.
[0031] FIG. 2 shows, as an exemplary embodiment of the invention, a
longitudinal section through an upper half of a magnetic resonance
device with a substantially tunnel-like patient acceptance chamber,
wherein for clarity again only the components in the section plane
are shown.
[0032] To generate an optimally homogenous static basic magnetic
field in the patient acceptance chamber, the magnetic resonance
device has a basic field magnet 210 with superconducting primary
coils 214 and likewise superconducting shielding coils 215
associated with the primary coils 214.
[0033] Furthermore, to generate rapidly switchable gradient fields,
the magnetic resonance device has an essentially hollow-cylindrical
gradient coil system 220 with a first transverse gradient coil 221,
a second transverse gradient coil 222, a longitudinal gradient coil
223, and gradient shielding coils 225, 226 and 227 associated with
the gradient coils 221, 222 and 223. The conductor arrangements of
the gradient coils 221, 222 and 223 are designed such that a middle
region of the gradient coil system 220 is free of conductors of the
gradient coils 221, 222 and 223, in which is arranged a
radio-frequency antenna element 240 of the magnetic resonance
device. The conductors of the gradient coils 221, 222 and 223
arranged on both sides of the middle region are surrounded by thin
metallic radio-frequency shields 231 and 232. The RF antenna
element 240 can either form an RF antenna by itself, or it can be
part of an RF antenna together with both RF shields 231 and 232. In
addition, the RF antenna element 140 and the RF shields 231 and 232
connected with one another in terms of radio-frequency.
[0034] The longitudinal gradient primary coil 223 formed by two
coils fashioned substantially like solenoids exhibits, from the
housing outward, a minimum with regard to its current density in
the aforementioned middle region, such that its free-of-conductors
design is unproblematic in this middle region. The essentially
transverse gradient coils 221 and 22 formed by four saddle coils
generally carry a current in the circumferential direction in this
middle region. Particularly, in the case of transverse gradient
coils with comparatively slight longitudinal extent, it is
necessary for realization of optimally linear gradient fields to
disperse the current in the middle region such that a minimum or
even a weakly developed reverse current density results. In the
illustrated design this can explicitly be set to zero such that a
middle region free of conductors is obtained. Given a longitudinal
extent of the gradient coil system 220 by approximately less than
one and a half times its diameter, the middle region can exhibit,
for example, a longitudinal extent of 12 cm.
[0035] The conductors of the gradient coils 221, 222 and 223
arranged on both sides of the middle region are, as stated,
surrounded by the metallic radio-frequency shields 231 and 232,
respectively. The radio-frequency shields 231 and 232 can carry a
high-frequency current and omit the conductor-free middle region.
Both radio-frequency shields 231 and 232 are provided in a known
manner with capacitively bridged gaps in order to keep the eddy
currents induced in the radio-frequency shield 231 and 232 by the
time-variable gradient fields small.
[0036] The short radio-frequency antenna element 240 arranged in
the middle region lies on a cylinder radius that is not smaller
than the inner radius of the gradient coil system 220. In contrast
to the conventional solutions, the radio-frequency antenna formed
by the RF antenna element 240 takes away no space within the
patient acceptance chamber. The field lines 249 of the
radio-frequency field that can be generated with the
radio-frequency antenna close within the gradient coil system 220
in a field return space 228 outside of the gradient coils 221, 222
and 223. A return of the gradient fields 229 also ensues in the
fields return space 228. Thus at least parts of the gradient coil
system 220 are used for the return of the radio-frequency field.
The radio-frequency shields 231 and 232 can form a part of the
current path of the radio-frequency antenna 240. An external
restriction of the return of the radio-frequency field first ensues
on the radio-frequency shield 223 associated with the gradient
shielding coils 225, 226 and 227. The RF shield 233 thus extends
radially outside of the antenna element 240. The RF field is
strongest in this region. In order to more clearly delimit the
field return space 233, the RF shield 233 preferably is elongated
on both sides in the axial direction, such that it extends in
regions disposed radially outwardly of the gradient coils 221, 222,
223. The symbols and .orgate. used to represent the field lines are
explained in FIG. 1. That which is specified in FIG. 1 for the
field lines 119 and 129 is valid for the field lines 219 of the
basic magnetic field and the field lines 229 of the second
transversal gradient field of FIG. 2,
[0037] FIG. 3 shows a magnetic resonance device with the components
of the MR device from FIG. 2, wherein a field return space 229' is
additionally shielded from radio-frequency in the axial direction.
This ensues by RF shielding side walls 234 that connect both ends
of the RF shield 233 with the RF shields 231, 232. The field return
space 228' is shielded from radio-frequency up to the middle
region, i.e. it is surrounded with RF shields up to the middle
region. This has the advantage that primary gradient coils 221,
222, 223 can be connected in series with the corresponding
shielding coils 225, 226, 227 without creating interactions of the
RF field with the connecting electrical conductors 224. The
expansion of the field return space 228' in the axial direction can
be optimized dependent on the efficiency of the RF antenna, taking
into account that a too-high magnetic field energy in the field
return space 228' that is too large can have a disadvantageous
effect on the efficiency.
[0038] As a further exemplary embodiment of the invention, FIG. 4
shows a longitudinal section through an upper half of a magnetic
resonance device with an essentially tunnel-like patient acceptance
chamber, wherein for clarity again only the components in the
section plane are shown. The magnetic resonance device has a
substantially hollow-cylindrical basic field magnet 310 with
superconducting primary and shielding coils 314 and 315, with a
barrel-shaped electrically-conductive vacuum vessel 312 of the
basic field magnet 310 in the region of the cavity to convert
[implement] the concept of the previously mentioned in German OS
101 56 770.
[0039] A gradient coil system 320 formed by two hollow-cylindrical
halves separated from one another is arranged in the cavity. The
gradient coil system 320 includes, from the inside out, a
longitudinal gradient coil 3232, a first transverse gradient coil
321, and a second transverse gradient coil 322. The sub-coils of
the gradient coils 321, 322 and 323 are, in each half, completely
enclosed by the radio-frequency shields 331 and 332. Analogous to
FIG. 2 or 3, a radio-frequency antenna element 340 is attached
between the halves of the gradient coil system 320. A sufficiently
large field return space 328 thus is available between the gradient
coil system 320 and the vacuum vessel 312 to close the field lines
349 of the radio-frequency field that is generated with the
radio-frequency antenna formed by the radio-frequency antenna
element 340. The vacuum vessel 312 either is fashioned as an RF
shield 333 on the side associated with the field return space 328,
or such an RF shield 333 is attached to it. The RF shield
preferably extends over the barrel-shaped bulge in order to
optimally shield the basic field magnet 310. The description for
the field lines 119 and 129 in FIG. 1 is valid for the field lines
319 of the basic magnetic field and the field lines 329 of the
second transversal gradient field of FIG. 4.
[0040] FIG. 5 shows a magnetic resonance device with the components
of the MR device from FIG. 4, wherein the gradient system 320 and
the barrel-shaped basic field magnet system 310 lie so close to one
another that the radio-frequency shield 333 and the first and
second radio-frequency shields 331 and 332 are connected with one
another in terms of radio-frequency. A field return space 328'
shielded from radio-frequency up to the middle region thereby is
achieved.
[0041] As a further exemplary embodiment of the invention, FIG. 6
shows in a perspective view a radio-frequency antenna 240 or 340
formed as a birdcage antenna disposed between the radio-frequency
shields 231 and 232 or 331 and 332. FIG. 7 shows, in a perspective
view as a further exemplary embodiment of the invention, a
radio-frequency antenna 240 or 340 formed as an array antenna
disposed between the radio-frequency shields 231 and 232 or 331 and
332.
[0042] Although modifications and changes may be suggested by those
skilled in the art, it is the intention of the inventors to embody
within the patent warranted hereon all changes and modifications as
reasonably and properly come within the scope of their contribution
to the art.
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